1. Field of the Invention
The present invention relates generally to structural wood panels and other molded products composed of cellulosic particles held together by an organic resin binder. More particularly, it relates to the selection of resin and other additives that will confer on such products enhanced resistance to fire.
2. Description of the Prior Art
The manufacture of wooden panels and other cellulosic articles from particles bonded together by a resin is well established in the art and encompasses products ranging from panels made of oriented wood strands embedded in a novolac type resin (U.S. Pat. Nos. 3,164,511; 3,202,743; and 3,478,861) to articles made from cellulosic particles such as wood flour molded with a thermosetting resin into useful shapes such as cups, trays, and plates. In the latter process, it has been found advantageous at times to replace the resin with about 1 to 2% by weight of the salt of a weak acid, e.g., ammonium oxalate, in order to avoid recourse to temperatures higher than 200.degree. C. during the molding process.
Another aspect of the prior art has been to provide these compounded cellulosic structures, including plywood panels, with increased fire resistance so that they can better serve the building industry. Of the numerous approaches used to achieve this goal, suffice it to mention here a few illustrative processes such as the impregnation of fire retarding substances into combustible paneling, the applications of a fire resistant coating on a surface to be protected, the use of specially treated binders, and the like. Thus, plywood structures have been treated with mixtures of sodium silicate, ammonium sulfate, urea, and other compounds, for fireproofing purposes (U.S. Pat. No. 3,811,992). Wooden panels have been protected by applying to them various layers of water soluble and water insoluble silicates (U.S. Pat. No. 3,663,355). Others have approached the problem of flame-retardance in fibrous sheet material such as textiles, by distributing the retarding agent, e.g., diammonium phosphate, in a pattern of varying concentration along the surface of the sheet (U.S. Pat. No. 3,767,452).
In the present instance, the objective has been the development of a method which would impart greatly improved fire resistance to conventional composites based on cellulosic substances. Another object has been to identify materials and combination of materials which could render such a method possible. A further object has been to upgrade the fire resistance of conventional wooden paneling sufficiently so that its use may be extended to areas from which it is now kept because of its flammability.